(1) Field of the Invention
The present invention relates to the field of NQR detection and more specifically to apparatus and methods for improving the echo signal of NQR detection.
(2) Description of Related Art Including Information Disclosed Under 37 C.F.R. 1.97 and 1.98
Terrorist use of high explosives against people in aircraft and buildings has unfortunately become a fact of life. To date there are no commercial explosives detection systems which can reliably detect threat quantities of military explosive against a background of more benign materials. The essentially free flow of contraband narcotics in this country, arguably a less dramatic problem, has greater consequences within the US. There are, likewise, no good ways to check for narcotics in a rapid, accurate and noninvasive fashion.
A major problem with NQR detection is that the radio-frequency (RF) pulses used to generate NQR signals can also generate from benign material non-NQR signals which can be mistaken for or mask true NQR signals. Examples of such spurious signals include magnetoacoustic ringing in spring mechanisms and piezoelectric ringing in quartz crystals. Furthermore, in the unshielded environment of landmine detectors radio frequency interference (RFI) from radio stations can also hide or mimic a true NQR signal. Spurious ringing and RFI can be much stronger than an NQR signal and detecting explosives in the presence of such interference is a major challenge.
Many materials containing quadrupolar nuclei are most efficiently detected by NQR techniques that employ a multiple RF pulse sequence composed of a preparatory pulse followed by a train of equally spaced refocusing pulses. The preparatory pulse generates a bulk magnetization from the quadrupolar nuclei and that magnetization is refocused at regular intervals by the pulse train. The resulting NQR signal typically occurs as an echo centered midway between each pair of refocusing pulses and the echoes are summed in order to improve the signal-to-noise ratio.
However, spurious signals such as acoustic ringing will also be amplified by such echo summation. Standard techniques for eliminating such ringing involve modulating the phases of the preparatory and/or refocusing pulses in a way that modulates the NQR signal differently than it modulates the ringing. For example, if one could invert the ringing after every other pulse but leave the NQR signal unchanged (this is difficult in practice) then a summation of all echoes would yield the full NQR signal and be free of ringing. Unfortunately, most of those schemes reduce the NQR signal or only partially reduce the ringing signal (or both).
The spin-locked spin-echo (SLSE) pulse sequence is well suited for detecting explosives like TNT and PETN which have a long NQR spin-lattice relaxation time (T1). In that pulse sequence the time interval between the preparatory pulse and the first pulse in the pulse train is one-half the pulse separation in the train, although sometimes the preparatory pulse interval is varied slightly to compensate for finite pulse length, filter delays and other effects that shift the position of the observed echo. The signal-to-noise ratio obtained in each scan is significantly enhanced by summing the train of echoes generated by the pulse sequence.
Unfortunately, spurious ringing is coherent with the pulses and, as a result, is also enhanced by echo summation. Spurious ringing can be cancelled to some extent by repeating the SLSE sequence with an inverted preparatory pulse as taught by Smith (PCT WO 96/26453). That inverts the NQR signal but not the ringing and proper summation of the two scans will enhance the NQR signal and reduce the ringing.
However, in order to maximize the NQR signal the scans must be separated by intervals greater than T1, which can be several seconds. During those waiting intervals the properties of the material that is ringing, and therefore the ringing itself, can change due to RF heating for example. This makes it impossible to cancel the ringing entirely. Because RFI is not coherent with the pulses a multiple scan detection is as likely to enhance RFI interference as to suppress it. To be successful, a cancellation scheme must work on time scales for which ringing and RFI are essentially constant.
Barrall (U.S. Pat. No. 6,392,408) discusses a modified SLSE pulse sequence which periodically inverts the NQR signal (but not the ringing) during the refocusing pulse train such that summing the echoes in a manner that retains the NQR signal also cancels the ringing. However, the inversions typically occur at intervals of hundreds of milliseconds and the nature of the ringing can change in those intervals making its cancellation incomplete. RFI can change between inversions even more than the ringing and, as a consequence, this method is not effective at cancelling RFI. Inversions are also imperfect and a substantial amount of NQR signal is usually lost.
It is well known in NMR (Bloom), and has been demonstrated in NQR (Smith, PCT WO 93/11441), that a sequence of RF pulses spaced unequally will create a host of echoes within the pulse intervals. What is discussed by Bloom, but not explicitly acknowledged by Smith, is that the total available signal within an interval is constant and spreading an intense echo at the center of an interval into myriads of subsidiary echoes does not per se improve the signal-to-noise ratio (SNR). In fact, such a procedure usually reduces the SNR since the signal is spread out through the large interval necessary for the observation of multiple echoes.
Another pulse sequence that is sometimes used for NQR explosives detection is called MLEV-16. Like the SLSE pulse sequence it consists of a preparatory pulse followed by a train of equally spaced refocusing pulses. In the standard form of the pulse sequence the preparatory pulse interval is also approximately one-half of the refocusing pulse interval. But the phases of the RF pulses in the MLEV-16 pulse sequence alternate in such a way that summing the echoes produces a net NQR signal but no net ringing.
Ringing cancels quite well in this pulse sequence because cancellation occurs within each 16-pulse cycle (on the order of tens, rather than hundreds or thousands, of milliseconds). Unfortunately, the net NQR signal for the MLEV-16 pulse sequence is approximately 50% smaller than that produced by a comparable SLSE pulse sequence. The SLSE and MLEV-16 pulse sequences each have two different pulse intervals (preparatory and refocusing) and the standard forms of both pulse sequences generate two types of echoes that occur at the center of each refocusing pulse interval. However, the echoes produced by MLEV-16 interfere more destructively than those produced by SLSE.